1. Field of the Invention
The invention relates to the field of digital transmission of voice and data, particularly in conjunction with telephone station sets.
2. Prior Art
Most telephones are connected to a central office or private branch exchange through a pair of leads referred to as a "twisted pair". These twisted pairs represent an enormous capital investment and as such cannot practically be changed. Analog (voice) signals, signalling information and ringing current are all transmitted over this single pair of wires. The twisted pairs are still preferred by installers (for example, over coaxial cable) even for new office building construction because they are very easy (and inexpensive) to install.
Today, it is not uncommon to use digital formats for switching voice signals and for transmitting voice signals over long distances where the twisted pairs are not used. The twisted pairs are relatively poor for transmitting digital signals, thus much of the telephone network has been confined to analog signals. Significant amount of high frequency distortion is inherent in the twisted pairs which deforms the leading and trailing edges of digital signals. In a typical twisted pair installation, there is also significant cross coupling between adjacent pairs, power transients and other problems resulting in signal being lost.
The present invention provides circuits and protocol for the transmission of digital signals over twisted pairs. The present invention, in conjunction with the twisted pairs, provides a digital link between a private branch exchange (computerized branch exchange) and telephone station sets, including sophisticated sets having displays and a link with a variety of data or video terminals. The link allows the transfer of both data and voice (in digital form) over a single twisted pair. The circuits of the present invention along with the protocol have been specifically developed to deal with the problems of digital transmission over twisted pairs.